Al-Zn-Mg-Ag high-strength alloy for aerospace and automotive castings

ABSTRACT

An aluminum casting alloy comprises, in weight percent: about 4-5% Zn; about 1-3% Mg; about 0-1% Cu; less than about 0.3% Si; less than about 0.12% Fe; less than about 0.5% Mn; about 0.01-0.05 wt % B; less than about 0.15% Ti; about 0.05-0.2% Zr; about 0.1-0.5% Ag; no more than about 0.05% each miscellaneous element or impurity; no more than about 0.15% total miscellaneous elements or impurities; balance aluminum. The alloy may advantageously be used in either T5 or T6 tempers.

This application claims benefits and priority of U.S. provisional application Ser. No. 60/684,513 filed May 25, 2005.

FIELD OF THE INVENTION

The present invention relates to alloy compositions and, more particularly, it relates to aluminum casting alloys for automotive and aerospace applications.

BACKGROUND OF THE INVENTION

Cast aluminum parts are widely used in the aerospace and automotive industries to reduce weight. The most common cast alloy used, Al—Si7—Mg, has well established strength limits. At present, cast materials in A356.0, the most commonly used Al—Si7—Mg alloy can reliably guarantee Ultimate Tensile Strength of 290 MPa, Tensile Yield Strength of 220 MPa with elongations of 8% or greater. The Al—Si7—Mg type high-strength D357 alloy can only guarantee Ultimate Tensile Strength of 350 MPa, tensile Yield Strength of 280 MPa with elongations of 5% or greater. In order to obtain lighter weight parts, higher strength materials are needed with established material properties for design. A variety of aluminum alloys, mainly wrought alloys, exhibit higher strength. The challenge in casting these alloys has been the tendency to form hot tears during solidification. Hot tears are macroscopic fissures in a casting as a result of stress and the associated strain, generated during cooling, at a temperature above the non-equilibrium solidus. In most cases, the castings cannot be salvaged for further processing because of the hot tears. Therefore, these wrought alloys are not suitable for use as casting alloys.

An associated cost with use of the Al—Si7—Mg alloys is the requirement for solution heat treatment (SHT). The heat treatment process requires expensive capital equipment, is often a source of problems in operation and control, engenders costs for both operation, control and testing, and has the potential to distort parts during the solution heat treatment process. While solution treatment is generally necessary for peak strength, it is a process with cost and operational disadvantages.

Eliminating solution heat treatment and quench to achieve adequate mechanical properties is always of interest in aerospace and automotive applications. The components targeted have minimum wall thicknesses of 1.5 mm. The SHT and quench not only add production cost but also create distortion due to creep at SHT temperature and residual stress generated by quenching. The distortion in large complex parts is very difficult if not impossible to correct. Therefore, it is preferred to have an alloy which can be used with only an artificial ageing step, referred to as T5 temper.

INTRODUCTION TO THE INVENTION

The invention is an Al—Zn—Mg base alloy for investment, low pressure or gravity permanent or semi-permanent mold, squeeze, high pressure die or sand mold casting with the following composition ranges, all in weight percent:

-   Zn: about 4 to about 5%; -   Mg: about 1 to about 3%; -   Cu: up to about 1%; -   Si: less than about 0.3%; -   Fe: less than about 0.12%; -   Mn: less than about 0.5%; -   B: about 0.01 to about 0.05%; -   Ti: less than about 0.15%; -   Zr: about 0.05 to about 0.2%; -   Ag: about 0.1 to about 0.5%; -   no more than 0.05% each miscellaneous element or impurity; -   no more than 0.15% total miscellaneous elements or impurities; Al:     remainder.

The alloy may be used in either T5 or T6 temper. The material in T5 condition can achieve mechanical properties higher than the expected from A356.0-T6, and close to the properties of D357-T6 alloy. In T6 condition, the material can achieve mechanical properties higher than those of D357-T6 high-strength alloy.

The invention also includes methods of making castings of the alloy, and alloys so produced.

SUMMARY OF THE INVENTION

In one aspect, the present invention is an aluminum casting alloy including, in weight percent:

-   about 4 to about 5% Zn; -   about 1 to about 3% Mg; -   up to about 1% Cu; -   less than about 0.3% Si; -   less than 0.12% Fe; -   less than about 0.5% Mn; -   about 0.01 to about 0.05 wt % B; -   less than about 0.15% Ti; -   about 0.05 to about 0.2% Zr; -   about 0.1 to about 0.5% Ag; -   no more than 0.05% each miscellaneous element or impurity; -   no more than 0.15% total miscellaneous elements or -   impurities; and -   balance aluminum.

In another aspect, the invention is a method of making an aluminum alloy casting, the method including:

preparing an aluminum alloy melt, said melt including the following elements in the concentrations shown:

-   about 4 to about 5% Zn; -   about 1 to about 3% Mg; -   up to about 1% Cu; -   less than about 0.3% Si; -   less than 0.12% Fe; -   less than about 0.5% Mn; -   about 0.01 to about 0.05 wt % B; -   less than about 0.15% Ti; -   about 0.05 to about 0.2% Zr; -   about 0.1 to about 0.5% Ag; -   no more than 0.05% each miscellaneous element or impurity; -   no more than 0.15% total miscellaneous elements or -   impurities balance aluminum; -   casting at least a portion of said melt in a mold -   configured to form said casting; and -   removing said casting from said mold.

In another aspect, the invention is an aluminum alloy casting, said casting including the following elements in the concentrations shown:

-   about 4 to about 5% Zn; -   about 1 to about 3% Mg; -   up to about 1% Cu; -   less than about 0.3% Si; -   less than 0.12% Fe; -   less than about 0.5% Mn; -   about 0.01 to about 0.05 wt % B; -   less than about 0.15% Ti; -   about 0.05 to about 0.2% Zr; -   about 0.1 to about 0.5% Ag; -   no more than 0.05% each miscellaneous element or impurity; -   no more than 0.15% total miscellaneous elements or -   impurities; and -   balance aluminum.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows T5 mechanical properties for samples of various alloys solidified under slow cooling rate (0.3° C./sec).

FIG. 2 shows T6 mechanical properties for samples of various alloys solidified at slow cooling rate (0.3° C./sec).

FIG. 3 shows T5 mechanical properties of various alloys in comparison to T5 properties of the Al-4.5 Zn-1.2 Mg Alloy.

FIG. 4 shows T6 mechanical properties of various alloys in comparison to T6 properties of the Al-4.5 Zn-1.2 Mg Alloy.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The above mentioned alloy and post casting method were evaluated in labscale. The alloys were cast in a directional solidification (DS) mold for mechanical properties evaluation. The casting from the DS mold possesses microstructure from various cross-sections which were solidified at different solidification rates. The casting was heat treated in T5 and T6 condition. The tensile properties were evaluated. Table 1 lists the tested Al alloy compositions in weight % with Al being the balance. The alloys containing silver were in accordance with the present invention. The other alloys are included in the table and the figures for comparison purposes. TABLE 1 Alloy Composition Composition Alloy Zn Mg Cu Si Ti B Zr Ag AO 4.5 1.2 — — 0.06 0.02 — — A1 4.5 1.2 0.25 — 0.06 0.02 — — A2 4.5 1.2 0.25 — 0.06 0.02 0.12 — A3 4.5 1.2 — — 0.06 0.02 — 0.3 A4 4.5 1.2 — 0.4 0.06 0.02 — — A5 4.5 1.2 — — 0.06 0.02 0.13 0.3 A6 4.5 1.5 — — 0.06 0.02 0.13 0.3

Table 2 shows the influence of different alloying elements on the T5 and T6 properties of the Al-4.5 Zn-1.2 Mg alloy at slow cooling rate (0.3° C./sec). FIG. 1 represents graphically the mechanical properties for various compositions at T5 conditions, while the mechanical properties at T6 condition are shown in FIG. 2. TABLE 2 Mechanical Properties Under T5 and T6 Conditions some samples solidified at cooling rates of - 0.30° C./sec. T5 T6 alloy Tensile Yield Elong. Tensile Yield Elong. A0: Al—1.2Mg—4.5Zn 266 192.5 10 275 223.5 10 265.5 198 8 268 222.5 10 A1: Al—1.2Mg—4.5Zn—0.25Cu 285.5 207 10 269.5 210.5 10 287 205 12 276 210.5 14 A2: Al—1.2Mg—4.5Zn—0.25Cu—0.125Zr 295 229 4 338.5 278.5 12 298.5 227.5 4 329.5 266 10 A3: Al—1.2Mg—4.5Zn—0.3Ag 308 241.5 10 351 309 12 311.5 248 10 351 308 14 A4: Al—1.2Mg—4.5Zn—0.4Si 238 174.5 10 309 230.5 16 238.5 173.5 10 312.5 233 16 A5: Al—1.2Mg—4.5Zn—0.3Ag—0.13Zr 322 266 10 358 317 12 322 265 10 358 317 12 A6: Al—1.5Mg—4.5Zn—0.3Ag—0.13Zr 358.5 299 11 362.5 326 8 358 298.5 12 361.5 327 8

It is noted that the best mechanical properties in Table 2 above are for the fourth, sixth and seventh (A3, A5, A6) of the alloys cited in Table 1. These alloys all contain silver, and their compositions fall within the composition range of the present invention.

The results for the various alloys after T5 heat treatment are plotted in FIG. 1 and the results after T6 heat treatment are plotted in FIG. 2. It is noted that the zinc concentration, which, in all cases, is 4.5%, has been omitted from the legend for the abscissa in these figures.

In FIG. 1, it is seen that the highest tensile yield strength (TYS), ultimate tensile strength (UTS), and elongation (E) in T5 temper were obtained for the alloy Al-4.5% Zn-1.5% Mg-0.3% Ag. The next best was for the alloy Al-4.5% Zn-1.2% Mg-0.3% Ag. These alloys fall within the composition range of the present invention.

In FIG. 2, it is seen that the highest yield strength and ultimate tensile strength for samples in T6 temper were obtained for the alloys of the present invention cited in the preceding paragraph. However, a higher elongation was obtained for the alloy Al-4.5% Zn-1.2% Mg-0.4% Si.

FIG. 3 presents tensile yield strength, ultimate tensile strength and elongation for the various alloys in comparison to the values for alloy Al-4.5% Zn-1.2% Mg. The values shown are for the samples after T5 temper. The abscissa presents the increase or decrease (negative values) of the cited values in comparison to the values for Al-4.5% Zn-1.2% Mg. For example, the ultimate tensile strength of the alloy Al-4.5% Zn-1.5% Mg-0.3% Ag-0.13% Zr is 54% greater than the corresponding value for Al-4.5% Zn-1.2% Mg.

FIG. 4 presents similar data for the various alloys after T6 temper.

Further investment casting tests were conducted using an invention alloy pursuant to the invention having a composition, in weight %, of 0.07% Si, 0.05% Fe, 0.30% Cu, 0.04% Mn, 1.61% Mg, 4.17% Zn, 0.14% Ti, 0.018% B, 0.13% Zr, 0.3% Ag, and balance Al and also using a “reference” alloy having a composition, in weight %, of 0.04% Si, 0.05% Fe, 0.29% Cu, 0.04% Mn, 1.75% Mg, 4.21% Zn, 0.14% Ti, 0.023% B, 0.13% Zr, and 0% Ag, and balance Al. The invention alloy and the reference alloy were investment cast to make two castings of each alloy composition under similar conditions (melt casting temperature of 740 degrees C. and mold temperature of 800 degrees C. using the Sophia process and air cooled) and then the resultant investment castings were heat treated to T6 temper.

In the T6 temper condition, the invention alloy investment castings exhibited mechanical properties as follows:

-   Casting No.1 -55.1 TYS (ksi), 61.1 UTS (ksi), and 12% E -   Casting No.2 -54.5 TYS (ksi), 60.2 UTS (ksi), and 10% E

The reference alloy investment castings exhibited mechanical properties as follows:

-   Casting No.1 -51.5 TYS (ksi), 57.8 UTS (ksi), and 9% E -   Casting No.2 -49.8 TYS (ksi), 56.1 UTS (ksi), and 9% E

It is apparent that higher strength and elongation were observed for the invention alloy investment castings and demonstrates the benefits of inclusion of Ag in the invention alloy.

It will be readily appreciated by those skilled in the art that modifications may be made to the invention without departing from the concepts disclosed in the foregoing description. Such modifications are to be considered as included within the following claims unless the claims, by their language, expressly state otherwise. Accordingly, the particular embodiments described in detail herein are illustrative only and are not limiting to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof. 

1. An aluminum casting alloy comprising, in weight percent: about 4 to about 5% Zn; about 1 to about 3% Mg; up to about 1% Cu; less than about 0.3% Si; less than about 0.12% Fe; less than about 0.5% Mn; about 0.01 to about 0.05 wt % B; less than about 0.15% Ti; about 0.05 to about 0.2% Zr; about 0.1 to about 0.5% Ag; no more than about 0.05% each miscellaneous element or impurity; no more than about 0.15% total miscellaneous elements or impurities; balance aluminum.
 2. An aluminum casting alloy according to claim 1 wherein a concentration of said zinc is about 4.5%.
 3. An aluminum casting alloy according to claim 1 wherein a concentration of said magnesium is about 1.2%.
 4. An aluminum casting alloy according to claim 1 wherein a concentration of said titanium is about 0.06%.
 5. An aluminum casting alloy according to claim 1 wherein a concentration of said boron is about 0.02%.
 6. An aluminum casting alloy according to claim 1 wherein a concentration of said zirconium is about 0.13%.
 7. An aluminum casting alloy according to claim 1 wherein a concentration of said silver is about 0.3%.
 8. A method of making an aluminum alloy casting, said method comprising: preparing an aluminum alloy melt, said melt comprising, in weight percent: about 4 to about 5% Zn; about 1 to about 3% Mg; up to about 1% Cu; less than about 0.3% Si; less than about 0.12% Fe; less than about 0.5% Mn; about 0.01 to about 0.05 wt % B; less than about 0.15% Ti; about 0.05 to about 0.2% Zr; about 0.1 to about 0.5% Ag; no more than about 0.05% each miscellaneous element or impurity; no more than about 0.15% total miscellaneous elements or impurities; and balance aluminum; casting at least a portion of said melt in a mold configured to form said casting; and removing said casting from said mold.
 9. A method according to claim 8 further comprising subjecting said casting to an artificial ageing heat treatment.
 10. A method according to claim 8 further comprising subjecting said casting to a solution heat treatment.
 11. A method according to claim 10 further comprising subjecting said casting to an artificial ageing heat treatment following said solution heat treatment.
 12. A shaped aluminum alloy casting, said casting comprising, in weight percent: about 4 to about 5% Zn; about 1 to about 3% Mg; up to about 1% Cu; less than about 0.3% Si; less than about 0.12% Fe; less than about 0.5% Mn; about 0.01 to about 0.05 wt % B; less than about 0.15% Ti; about 0.05 to about 0.2% Zr; about 0.1 to about 0.5% Ag; no more than about 0.05% each miscellaneous element or impurity; no more than about 0.15% total miscellaneous elements or impurities balance aluminum.
 13. An aluminum alloy casting according to claim 12 wherein a concentration of said zinc is about 4.5%.
 14. An aluminum alloy casting according to claim 12 wherein a concentration of said magnesium is about 1.2%.
 15. An aluminum alloy casting according to claim 12 wherein a concentration of said titanium is about 0.06%.
 16. An aluminum alloy casting according to claim 12 wherein a concentration of said boron is about 0.02%.
 17. An aluminum alloy casting according to claim 12 wherein a concentration of said zirconium is about 0.13%.
 18. An aluminum alloy casting according to claim 12 wherein a concentration of said silver is about 0.3%. 